Datasheet

43
2486AA–AVR–02/2013
ATmega8(L)
Watchdog Timer The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1MHz. This is
the typical value at V
CC
= 5V. See characterization data for typical values at other V
CC
levels. By
controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as
shown in Table 17 on page 44. The WDR – Watchdog Reset – instruction resets the Watchdog
Timer. The Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs.
Eight different clock cycle periods can be selected to determine the reset period. If the reset
period expires without another Watchdog Reset, the ATmega8 resets and executes from the
Reset Vector. For timing details on the Watchdog Reset, refer to page 41.
To prevent unintentional disabling of the Watchdog, a special turn-off sequence must be fol-
lowed when the Watchdog is disabled. Refer to “Watchdog Timer Control Register – WDTCR”
for details.
Figure 20. Watchdog Timer
Watchdog Timer
Control Register –
WDTCR
• Bits 7..5 – Res: Reserved Bits
These bits are reserved bits in the ATmega8 and will always read as zero.
• Bit 4 – WDCE: Watchdog Change Enable
This bit must be set when the WDE bit is written to logic zero. Otherwise, the Watchdog will not
be disabled. Once written to one, hardware will clear this bit after four clock cycles. Refer to “Bit
3 – WDE: Watchdog Enable” on page 44for a Watchdog disable procedure. In Safety Level 1
and 2, this bit must also be set when changing the prescaler bits. See the Code Examples on
page 45.
WATCHDOG
OSCILLATOR
Bit 76543210
– – – WDCE WDE WDP2 WDP1 WDP0 WDTCR
Read/Write R R R R/W R/W R/W R/W R/W
Initial Value00000000